15w and 20w chargers are different in power, and the charging time will be different, which is the main difference between them.
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for example
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① Huawei P9 original charger DC output: 5V\/2A or 7.5V\/2A, power is 15W.
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2 Mi 8 original charger DC output: 5V\/3A, 9V\/2A, 12V\/1.5A, power is also 15W.
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③oppo R17 original charger DC output: 5V\/4A, this power is 20W. Therefore, if the power is different, the DC output voltage and current of the charger are different.
20w is 5w more power than 15w, and the wireless fast charge is fast